When dispensing gasoline to the tanks of individual vehicles at a retail service station, the gasoline typically is pumped from large storage tanks, which are often located underground. Such storage tanks, in turn, are refilled periodically from tanker trucks, which receive gasoline at a central distribution site and deliver it to individual service stations where it is pumped from the truck into the underground storage tanks.
Gasoline storage tanks at retail service stations typically are vented to the atmosphere so that air can be drawn into the tank to displace liquid gasoline as it is pumped from the tank and into the gas tanks of vehicles. As a result, the head space within the storage tank, i.e. the space above the surface of liquid gasoline in the tank, is progressively filled as the tank is emptied with a mixture of oxygen, nitrogen, and water vapor from the atmosphere, and highly concentrated gasoline vapor, which evaporates from the surface of liquid gasoline within the tank.
In the past, the gases and vapors within the head space of gasoline storage tanks was simply re-vented into the atmosphere each time the storage tank was filled from a tanker truck. In recent years, however, environmental concerns have lead to requirements that head space vapors within gasoline storage tanks be recovered when the tanks are refilled to prevent introduction of the gasoline vapors into the atmosphere. Usually, such vapors are simply directed through a recovery conduit into the head space of the tanker truck as liquid gasoline from the tanker truck is pumped into the storage tank. When the tanker truck has depleted its load of liquid gasoline and is filled with head space vapors collected from the storage tanks that were serviced, the tanker truck returns to the central distribution station or to a processing station. Here, the concentrated vaporous mixture is retrieved from the tanker truck for further processing.
In some instances, the retrieved vaporous mixture is simply burned to minimize the impact of its release into the atmosphere. In many instances, however, the vaporous mixture is processed to condense the gasoline vapor to liquid gasoline and thus recover the liquid gasoline from the mixture. The recovered liquid gasoline can then be redistributed to individual service stations for sale.
Various techniques are available for recovering the gasoline vapor from such mixtures. Carbon adsorption regeneration and mechanical refrigeration techniques typically involve cooling the mixture to a temperature below the condensation point of gasoline, whereby the gasoline vapor condenses into liquid gasoline which can be collected for redistribution. One such gasoline recovery system is illustrated and discussed in an article by A. H. Hall entitled "Operational Experience of the BOC Liquid Nitrogen Condensation Vapour Recovery Unit".
Although liquid gasoline recovery systems have been somewhat successful in recovering gasoline from vaporous mixtures, they nevertheless have been plagued with numerous problems and shortcomings. For example, vapor mixtures recovered from gasoline storage tanks commonly include high concentrations of water vapor and oxygen from the atmosphere. Since water vapor has a much higher condensation temperature than gasoline, it tends to condense out of the mixture long before condensation of gasoline begins to occur. As a consequence, prior art recovery systems typically include a pre-cooler wherein the mixture is pre-cooled to a temperature between the condensation points of water and gasoline in an attempt to condense the water out of the mixture. The precondensed water is then collected and drained from the system before the condensation of gasoline is commenced. In addition, carbon adsorption recovery systems suffer from reduced efficiency due to active molecular adsorption sites being occupied by water vapor instead of hydrocarbon material.
Even with such pre-cooling, some water vapor remains in the mixture. As a result, during further cooling of the mixture to condense the gasoline vapor, this water freezes and the resulting small ice crystals tend to destroy the pumps, seals, and valves of the system. Furthermore, when the condensed gasoline returns to normal temperatures, the ice crystals melt and mix with the gasoline, thus reducing the quality of the gasoline condensate and requiring further gravity separation techniques. Also, the condensation of water vapor from the mixture requires energy, which otherwise might be used in condensing the gasoline vapor itself.
In addition to problems associated with water vapor in the mixture, oxygen in the mixture can also cause problems. When liquid nitrogen is used as a condensing coolant, for example, there is a risk that the oxygen within the mixture will undergo a phase change to its liquid state. Naturally, intimate contact between highly volatile gasoline and liquid oxygen can create an extremely dangerous explosive condition. In addition, the mere presence of oxygen gas in the initial vapor mixture creates a potential for explosion that must be seriously considered when designing tanker trucks and processing equipment.
Accordingly, a continuing and heretofore unaddressed need exists for a gasoline vapor recovery methodology wherein the above discussed problems associated with water vapor and oxygen in the mixture are eliminated, where energy is not wasted condensing water out of the mixture, and wherein high quality liquid gasoline is recovered from the mixture economically and with minimum system complexity. It is to the provision of such a methodology that the present invention is primarily directed.